1. Field of the Invention
The invention is directed to a semiconductor component having at least one pn junction, preferably a diode for applications in the power class. Diodes of this type are employed by way of example in combination with power switches, especially power transistors, as so-called freewheeling diodes.
2. Description of the Related Art
A known method of manufacturing semiconductor devices, including diodes, is described in European Patent No. EP 1 096 576 A1, which describes forming diodes in a layer sequence with a first zone having a first dopant and further zones having a second dopant. In this case, doping is understood to mean the arrangement of dopant atoms, of donors or acceptors, in a unit of volume; consequently, a concentration of dopant atoms per unit volume is defined therein. In this case, in accordance with the prior art, in the further zones the concentration of the second dopant increases continuously or discontinuously proceeding from the first zone. It is known that, in individual zones, among said further zones, the concentration of the doping may be constant; likewise, the concentration of the doping in one or more zones, can increase in a manner obeying an exponential function. What is essential, however, is that the concentration of the doping has a basic profile in accordance with FIG. 4 hereof.
FIG. 4 shows by way of example the concentration profile of the doping of a power diode according to the prior art. Here the illustration shows a first main area, (H1), to which a first, here a p-doped, zone (10) is adjacent. This is followed by an n−-doped second zone (20), which preferably has the constant basic concentration of the doping of the base material, here of a silicon wafer. Adjacent to second zone (20) is a first interface (G1) and a third, n-doped zone (30), which acts as a buffer layer. As last, a fourth zone (40) of the layer sequence there follows from an interface (G2) with an n+-doped zone and subsequently the second main area (H2) of the semiconductor component. Required metallizations of main areas (H1, H2) for contact purposes are not described any further here.
In accordance with the prior art, third zone (30) and fourth zone (40) have been produced by diffusion from the direction of second main area (H2). This also results in the profile of said dopings that is typical of diffusion methods. In accordance with the prior art, the p-type doping of first zone (10) is effected by means of diffusion methods from the direction of first main area (H1).
As mentioned above, power diodes of this type are employed as freewheeling diodes in antiparallel connection with power transistors. FIG. 5 shows the current and voltage profile across the diode in the case of switching on an antiparallel-connected power transistor, the diode being converted from the on state to the off state. In this case, the voltage (Ui) drop across the diode ideally rises to approximately the value of the voltage source. At the same time, the current (Ii) through the diode falls to zero. In the further course of the profile, the current flow becomes negative, or changes its direction, since here the pn junction is depleted of charge carriers. The current furthermore decreases to the reverse current of the diode.
In real operation, the current profile (IR) and voltage profile (UR) differ from the ideal due to parasitic inductances and a non-ideal component characteristic of the diode and the power transistor. As a result of a fast temporal change in the current after the reverse current peak (IS), overvoltages and resultant oscillations are induced in the real voltage profile (UR) and therefore also in the real current profile (IR). In this case, the physical effect is the fast field propagation of the electrical field in the diode. As a result of this, the positive and negative charge carriers are removed very rapidly, whereby current chopping takes place. Known buffer layers (30, cf. FIG. 4) here form a reserve of charge carriers in order to reduce the rate of removal and hence to slow down the current chopping.
The aforementioned oscillations (S1, S2) on the one hand limit the maximum possible switching speed of an arrangement comprising the diode and power transistor, which means a restriction of the functionality of power converters constructed therefrom. On the other hand, excessively high amplitudes of the oscillations (S1, S2) can destroy the diode itself. Therefore, there is a need in the art for a power diode that exhibits minimum oscillation and a small oscillation amplitude (S1, S2).